1. Field of the Invention
The present invention relates to a driving method, and more particularly, to a method and device for driving a light-emitting diode (LED) with an improved power factor and power utilization efficiency and a reduced electromagnetic interference.
2. Description of the Prior Art
As natural energy resources are being used up, some new substitute energy resources are proposed but cannot immediately answer and fulfill all human needs and requirements. Therefore, saving natural resources becomes a very important issue. Accordingly, many electronic devices or equipments are improved in the hope of decreasing power consumption. For example, assuming that a conventional tungsten lamp consumes 100 units of electric power, only 5 units of electric power is transformed into light and the rest of the electric power is transformed into the heat. Thus, the transforming efficiency of the conventional tungsten lamp is much less than satisfactory. Additionally, a heat sink or a cooler system may be required to dissipate the heat generated from the conventional tungsten lamp, which will consume additional electric power. The conventional lamp equipments have long suffered from the drawback of low power utilization efficiency. As the technology development and innovation in semiconductor industry have quickly advanced in the recent years, light-emitting diodes (LEDs) continue to gain popularity and are increasingly used in illumination application, taking advantage of their long service life and low power consumption.
As mentioned above, LEDs are advantageous in long service life, low power consumption and low waste heat generation. All of these advantages promote the development of LED lighting equipments. An LED is normally driven by a DC power source. As such, when the LED is connected to an AC power source, such as a mains electricity supply, it has to be provided with an LED driver circuit that converts the incoming AC power into a pulsed DC power, so that the LED can emit light upon receipt of the DC power.
As shown in FIG. 4(A), a conventional LED driver circuit 100 is connected to at least one LED 101 and adapted to receive an AC power VAC. The LED driver circuit 100 mainly comprises abridge rectifier 110 and a capacitor C. The bridge rectifier 110 is used to rectify the AC power VAC into a pulsed DC power, and then the capacitor C stabilizes the voltage of the pulsed DC power for driving the LED 101 to emit light. As shown in FIG. 4(B), a constant current circuit is further provided to maintain the current at a constant level, thereby stabilizing the brightness and chromaticity of the light emission from the LED. An inductive reactance element, such as a capacitor or an inductor, if present in the circuit, will make the voltage and current out of phase by a phase difference (θ), as shown in FIG. 4(C). Given the equation that PF(Power Factor)=V(Voltage)×I(Current)×cos θ, since the voltage and current is out of phase in this case, the presence of the phase difference (θ) causes a decrease in power factor which in turn results in an increase in power loss.
R.O.C. Patent No. 1220047, entitled “LED Driver Circuit,” discloses an LED driver circuit as shown in FIG. 5, which includes a power source 51, multiple current guiding control circuits 52 composed of one or more common-ground current control units 521, and a voltage detection circuit 53 for detecting the voltage level of the supplied power. The power source 51 is connected in series to one or more LED sets 54, each being composed of one or more LEDs. The current control units 521 of the current guiding control circuits 52 are sequentially connected to the N-electrodes (negative electrodes) of the respective LED sets 54. The voltage level of the positive part of the supplied power is detected by the voltage detection circuit 53. Then, one of the current guiding control circuits 52 is selectively placed in a conductive state according to the voltage level, whereby a suitable amount of the LED sets 54 are driven to emit light. Such a circuit design allows the voltage of the positive part of the supplied power to directly drive the LED sets, so that a maximum number of LED sets are driven to emit light according to the voltage level without coupling to a filtering capacitor, thereby achieving the purposes of efficiently utilizing electric power, increasing power factor and reducing power loss.
However, the driver circuit described above has the following drawbacks:
1. A voltage detection circuit is required for detecting the voltage level of the positive part of the supplied power, leading to a complicated circuit construction.
2. A selected one of current guiding control circuits is placed in a conductive state under the circumstance that the voltage detection circuit detects the incoming voltage level (the remaining current guiding control circuits are kept in an electrically disconnected state), thereby allowing a particular amount of LED sets to emit light. In the case of malfunction of the voltage detection circuit, none of the LED sets can be driven to emit light.